Whether you're painting walls yourself or paying a professional to do it for you, it pays to prepare in advance. If you don't, you're setting yourself up for a disappointing end result and possibly the need to do it all over again not too far in the future.
Usually, when preparing to paint, people give the walls a quick clean, a scrub, and maybe a light brushing with sandpaper, then get going. And, while this is sometimes all you need, there are things to look out for that could really ruin your eventual paintwork if you don't sort them. Be aware of any of these potential problems so you can deal with them before the painting begins:
Any greasy spots on walls can play havoc with paint adhesion, quickly leading to peeling or flaking spots once the paint dries. It should be easy to spot, and using one of the commercially available degreasing products will clean it up without much hassle. Grease is more common in kitchens, but things like blue tack can also leave greasy marks, so check thoroughly.
Even small holes can ruin the look of your new paint, so find them and fix them. If you're inspecting for them before you're able to fill them, circle any holes you find with a pencil so they're more obvious. Ordinary filler does the trick, but make sure you sand it smooth after it's set.
Surprisingly, a lot of people don't notice they've left picture hangings and other small fixtures on the walls until they have a brush in their hand about to start painting. Don't forget to fill in any holes left behind after you remove them; it's better to drill new ones after you paint.
Mould is not just unhygienic; it won't do much for your paintwork, either. It's often black, which is easy to spot, but not always. If you find any mould, clean it thoroughly with bleach, follow up with a soap and water wash, and make sure the wall is completely dry before you proceed.
If you have significant damp on your walls, unfortunately, it probably points to a larger problem. Sometimes fixing it is as simple as increasing your ventilation, but it could be leaking from somewhere. There's no point painting until the cause is found and fixed because the damp problem will just ruin the paint. Once you've isolated what's causing the damp, let the walls dry out before you paint, and check for damaged plaster.